keylight artifacts

i´m working on some greenscreen shots, comparing different keyers and settings.Keylight is giving me some headaches. It does a really good job on the edges and for the despilling, but it also introduces horrible artifacts within the image. I tried different settings to avoid it, but it seems, that only switching the keyer to “intermediate result” gets rid of those artifacts.

There doesn´t seem to be any information inside the image to justify such artefacts, the matte is clean it looks like there´s some problem in the despilling algorithm. One thing that improves the image is when I turn the replacement mode to “hardcolor”. Still you can see those artifacts, but it´s slightly better.

Any idea what could cause this problem? I also tried primatte, which is doing a pretty good job, but not as good as primatte would do concerning the edges.